1. Field of the Invention
The present invention relates to a planographic printing original plate which can be used as an offset printing master, and particularly to a planographic printing original plate (which can be formed by a so-called direct plate-making process) which can be directly inscribed based on digital signals of a computer or the like by using an infrared laser.
2. Description of the Related Art
In recent years, attention has been focused on a system in which a printing plate is directly formed based on digital signals in a computer or the like without using a lithographic film. In this system, lasers are useful as an exposure light source. High power and small-size lasers such as solid-state lasers and semiconductor lasers, which emit rays in regions from a near-infrared region to an infrared region, which lasers have recently been remarkably developed, are particularly useful.
As an image recording material suitable for writing using such lasers, for example, a positive-type image recording material disclosed in Japanese Patent Application Laid-Open (JP-A) No. 7-285,275 is used. The positive-type image recording material disclosed therein comprises a binder such as a cresol resin, a substance which absorbs light to generate heat, and a substance which is thermally decomposed like a quinone diazide and which, in a non-decomposed state, substantially reduces the solubility of the binder. When the positive-type image recording material is exposed to an infrared laser, in an exposed region of the positive image recording material, the substance which is provided to generate heat by absorbing light generates heat and the solubility is thereby revealed. Subsequently, when the positive-type image recording material is developed with an aqueous alkaline solution, only the above-described exposed region is dissolved and removed and a base surface is uncovered. In the above-described exposed region (non-image portions), a hydrophilic surface of the base is exposed. Accordingly, the exposed region retains water and forms an ink repellent portion. On the other hand, in a non-exposed region other than the aforementioned exposed region (image portions), a lipophilic photosensitive layer remains, and therefore, the non-exposed region becomes an ink receiving portion. As a result, desired image recording can be achieved.
However, not only such an image recording material but also positive-type photosensitive printing original plates conventionally widely used have the following problems. Namely, conventionally, an aluminum base material subjected to surface roughening, alkali etching treatment, or anodic oxidation treatment is used as a support. However, the surface of the aluminum support does not necessarily have sufficient hydrophilicity, and therefore, in the non-image region, an ink repulsive force is not sufficiently obtained, and the non-image region is thereby stained with ink (that is, smudging occurs). As a result, an image of high image quality is not obtained.
Further, heat generated in the exposed region (non-image portions) is absorbed into the aluminum support and thermal efficiency is low. Accordingly, in a development process, the solubility of the exposed region into an alkaline developing solution is not sufficiently obtained, and an oleophilic photosensitive layer is not dissolved and a portion thereof remains in the exposed region after the development processing, thereby causing the smudging. As a result, an image of high image quality is not obtained.
In order to solve the above-mentioned problem in which the smudging is caused, there has conventionally been known a technique in which the hydrophilic property on a surface of the aforementioned aluminum support is improved so as to allow the exposed region (non-image portions) to be made hydrophilic. However, when an oleophilic positive-type photosensitive layer is provided directly on the hydrophilic support, layer-to-layer adhesion between the support and the photosensitive layer is poor, and therefore, a problem arises in which the durability in printing (that is, an ability of normal printing) becomes deteriorated.
For this reason, only the exposed region (non-image portions) is made hydrophilic by using a developing solution containing silicate such as potassium silicate and sodium silicate during the development processing. However, when the developing solution containing silicate is used, solid matters originating from SiO2 is apt to be precipitated. Further, when a waste liquid of the developer is neutralized, gel originating from SiO2 may be generated.
Accordingly, there has been made a demand for a technique in which even if an oleophilic positive-type photosensitive layer is provided directly on a support of which surface is in advance made hydrophilic, the layer-to-layer adhesion between the support and the photosensitive layer and the durability in printing are both excellent, which results in no smudging being caused.
Under these circumstances, in U.S. Pat. No. 3,136,636, it is proposed that an intermediate layer comprising a water soluble polymer such as polyacrylic acid and carboxylmethylhydroxy ethyl cellulose is provided. In this case, a problem exists in which the durability in printing is not sufficiently obtained. Further, in U.S. Pat. No. 4,483,913, it is proposed that an intermediate layer comprising a quaternary ammonium compound such as poly(dimethyldiaryl ammonium chloride), but in this case, a problem exists in which the aforementioned smudging is caused.
In order to solve various problems in these conventional arts, the present inventors have realized a planographic printing plate which is excellent in the durability in printing and prevents the smudging by providing an intermediate layer which contains a polymeric compound having a specified structural unit such as p-vinyl benzoate (Japanese Patent Application Laid-Open (JP-A) No. 10-69,092).
The aforementioned intermediate layer itself has an excellent effect in improvement of the durability in printing, but there is room for further improvement with respect to the viewpoint of preventing occurrence of the smudging. Namely, for example, when the intermediate layer is combined with a support layer having excellent hydrophilicity, it has effects in that excellent durability in printing is obtained and the above-mentioned smudging does not occur. Therefore, there has been desired further improvement with respect to the effect that no smudging is caused, which is a function of the intermediate layer itself.
The present invention is provided to solve the aforementioned various problems in the conventional arts and to achieve the following object. Namely, an object of the present invention is to provide a planographic printing original plate which, with a processor or a printer being capable of being utilized as is, can be directly inscribed based on digital signals of a computer or the like and which has excellent durability in printing and causes no smudging.
The above-described object is achieved by a planographic printing original plate of the present invention. Namely, the planographic printing original plate of the present invention comprises an intermediate layer which contains a polymer including, as a monomer unit, at least a monomer having an acid group and a monomer having an onium group; and a photosensitive layer comprised of a positive-type photosensitive composition for an infrared laser which contains: (A) at least an alkali-soluble polymeric compound; (B) a compound which has a function to deteriorate the solubility of the alkali-soluble polymeric compound in an alkaline aqueous solution due to compatibility with the alkali-soluble polymeric compound, the function deteriorating by compound (B) being heated; and (C) a compound which generates heat by absorbing light, the intermediate layer and the photosensitive layer being formed sequentially on a support which has been subjected to a hydrophilizing treatment.
In the planographic printing original plate of the present invention, an aspect in which the polymer contained in the intermediate layer is a polymeric compound, an aspect in which the polymer is soluble in alkali water, an aspect in which the number average molecular weight (Mn) of the polymer is in the range of 300 to 5,000, and an aspect in which the polymer is obtained by using together a polymerization initiator and a chain transfer agent during radical polymerization are preferably provided. Further, an aspect in which the hydrophilizing treatment is a silicate treatment is also preferably provided.
In the planographic printing original plate of the present invention, since the intermediate layer which contains a polymer including, as a monomer unit, a monomer having an acid group and a monomer having an onium group is provided between a support and a photosensitive layer, as compared with a conventional planographic printing original plate in which an oleophilic photosensitive layer is directly formed on a support subjected to the hydrophilizing treatment, layer-to-layer adhesion between the support and the photosensitive layer is improved and the durability in printing remarkably improves.
Further, in the non-image portions, the acid group of the polymer contained in the intermediate layer is apt to be dissociated in an alkaline developing solution, and therefore, the polymer is dissolved and removed from the surface of the support. Moreover, the polymer has also the onium group and the solubility thereof is greater than that of a polymer having only an acid group, which polymer is contained in an intermediate layer of a planographic printing original plate described in JP-A No. 10-69,092. Accordingly, in the planographic printing original plate of the present invention, the surface of the support, which has been subjected to the hydrophilizing treatment in a development process, is apt to be uncovered and an effect of preventing occurrence of the smudging is remarkably improved.